In recent years, as personal computers become prevalent, as a presentation tool in business and other scenes, image projectors have been coming into wider and wider use that project images displayed on a display device (such as a liquid crystal device, a DMD, or the like) onto a screen. Moreover, an increasing demand is prospected for projectors that can be used as home-use projecting apparatuses (for example, projectors fit for realizing so-called home theaters or those fit for use with a digital television system). For such uses, single-panel or three-panel liquid crystal projectors and DMD projectors (having about 100,000 to 300,000 pixels) have conventionally been developed.
Projection optical systems for use in such projectors are proposed in U.S. Pat. Nos. 5,745,297 and 5,666,228, and Japanese Laid-open Patent Applications Nos. H10-142503 and H10-170824. The projection optical systems proposed in the above-mentioned patent applications have half angle of view of about 40.degree. and offer satisfactory optical performance as long as they are used to project images obtained from conventional display devices. However, these projection optical systems do not achieve sufficient reduction of aberrations (lateral chromatic aberration, in particular) as is required to properly project images obtained from higher-resolution display devices. On the other hand, as a projection optical system that offers higher projection performance, a retrofocus-type lens system is known that is proposed in Japanese Laid-open Patent Application No. H7-270680. This projection optical system is so designed that lateral chromatic aberration is suppressed by the use of a positive lens element made of anomalous-dispersion glass included in the rear lens unit. However, this projection optical system has, for example, half angle of view of about 20 to 28 degrees and an F number of 4.5. This fact shows that it fails to offer sufficiently high optical performance.
Nowadays, higher and higher image quality has been sought after in projectors than ever. For example, for presentation purposes, projectors having resolutions higher than 1024.times.768 dots (XGA) are preferred to those having resolutions of 800.times.600 dots (SVGA). Even in home-use projectors, horizontal resolutions higher than 400 scan lines are in demand, now that high-definition television services such as Japan's Hi-Vision have gone into operation. To satisfy such requirements for higher image quality, display devices such as liquid crystal devices or DMDs have come to offer increasingly high resolutions. In addition, to make projectors as compact as possible, display devices such as liquid crystal devices and DMDs are made smaller and smaller.
To achieve proper projection of images obtained from such a display device that has a smaller size and offers a higher resolution, it is essential to use a projection optical system that offers higher optical performance than ever. However, as noted previously, conventional projection optical systems do not offer sufficiently high optical performance (in particular, lateral chromatic aberration correction performance) as desired. Moreover, in particular, projection optical systems designed for use in home-use projectors need to be made as low-cost as possible. However, in conventional projection optical systems, higher optical performance cannot be achieved without increasing manufacturing cost.